Ok, here's a handy little tool that I created recently for testing URL parameters in the query. I guess you call these things 'bookmarklets' but it's essentially just a bunch of javascript that you can put in a bookmark and then execute whenever you like (on any page).
This particular bookmarklet parses and decodes all of the query parameters and provides a simple (and rather ugly) little UI to modify those parameters or add new ones. After you have hacked the params all to hell and back you can click a button to encode them all and see what happens.
Let me know if it works for you and/or if you find it useful. I found it EXTREMELY handy for testing complex query params and that's why I created it. Sure there's a bunch of tools out there that will do something similar but this is small, simple and fast. Duke Of URL has been tested on the latest FF, IE and Safari but let me know if you find any bugs.